The field of the invention relates to noninvasive cardiac monitoring, and more particularly to an all electrode-based system employing techniques of impedance cardiography and electrocardiography for measuring mechanical and electrical activity, respectively, of the myocardium for use in the diagnosis of coronary artery disease.
U.S. patent application Ser. No. 07/113,444 filed Oct. 28, 1987, describes a bioelectrical and bioimpedance surveillance system for measuring and analyzing physiological functions through the use of bioelectrical and bioimpedance signals. In the case of cardiac monitoring, the bioelectric signal comprises the electrocardiogram (EKG). As is well known, the EKG detects surface potentials which arise from electrical changes originating within the body, and more specifically from myocardial depolarization and repolarization waves.
Bioimpedance, on the other hand, relates to the electrical impedance of living tissues as measured when electrical energy is applied to the body. Specifically, radio frequency currents are injected into a particular body segment of interest and the potential across this segment is measured from which the bioimpedance signal is obtained. The bioimpedance signal is not of physiological origin, but it can measure physiological phenomena which exhibit varying impedance over a period of time. For monitoring cardiac function by bioimpedance, a low-level electric current is induced across the thorax and changes in the transthoracic impedance during the cardiac cycle are measured. The measured transthoracic signal contains respiratory and cardiac components. However, the cardiac components can be isolated by taking the first derivative of the impedance signal, which essentially filters out the lower frequency respiratory signal. The first derivative of the impedance signal, often referred to as the dZ/dt signal, is related to aortic blood velocity and acceleration as numerous clinical studies reported in the literature have demonstrated.
In the past the focus of those investigating the use of impedance cardiography was to use it to evaluate cardiac function by measuring stroke volume and cardiac output. The theory is based on the principle that because blood is a conductor of electricity, transthoracic electrical impedance changes during the cardiac cycle relate to the amount of blood ejected by the heart. Various empirical formulas have been used to estimate stroke volume from the impedance signal. See, for example, U.S. Pat. No. 4,450,527 to Sramek. The formulas for stroke volume are functions of ventricular ejection time, the electrical resistivity of the blood, distance between the thoracic electrodes, and geometric assumptions about the shape of the thorax. Although the reliability and reproducibility of stroke volume measurements by impedance cardiography have been good in specific instances, in other instances it has not been. In various subsets of patients (for example, in those with hypertension, in exercise studies, and the presence of rapid heartrates), the stroke volume as measured by impedance cardiography has been excessive or inaccurate. Due to such inaccuracies, as well as the use of empirical formulas, and a relative lack of correlation with other presently used noninvasive methods, impedance cardiography has not been widely accepted and is currently viewed primarily as an investigational technique.
The present invention takes a different approach to impedance cardiography. Rather than trying to derive precise quantitative values for parameters such as stroke volume, the methodology of the present invention is to make use of the information provided directly by the impedance cardiogram (dZ/dt). That is, the present inventors have conducted clinical studies which demonstrate that portions of the dZ/dt waveform, and changes thereto over time, directly correlate with cardiac function and dysfunction in much the same way that variations in the electrocardiogram have been correlated with cardiac condition.
The electrocardiogram provides an assessment of electrical function of the myocardium. Clinical studies have established that certain deviations of the electrocardiogram from the norm are suggestive of cardiac dysfunction. For example, it is now widely accepted that myocardial ischemia shows up as an excessive depression in the ST-segment portion of the electrocardiogram. Changes in the electrocardiogram which suggest the presence of ischemia may be misleading, however when unaccompanied by changes in cardiac mechanical function, often referred to as global ventricular performance. Concomitantly, changes in cardiac mechanical function can also occur without accompanying change in electrical activity, so that some episodes of ischemia remain undetected.
Other noninvasive techniques are known for measuring the mechanical function of the heart. For example, nuclear imaging (MUGA) has been used to measure exercise induced changes in ejection fraction. The amount of change, or lack of change in the ejection fraction has been correlated with global ventricular performance. This technique has several drawbacks, not the least of which is that it requires tagging the patient's blood with a radio nuclide, a radioactive material. Additionally, nuclear imaging is capital-intensive so that only large facilities such as hospitals can afford the equipment, and the procedures are costly to the patient. Further, the sensitivity and specificity of exercise-induced ejection fraction changes observed through nuclear imaging are not as high as one would like, presenting an undesirable number of false positives and false negatives.
Doppler echocardiography is another procedure for noninvasively determining global ventricular performance. This technique has been employed in an exercise tolerance test modality with varying success. Its drawbacks are that a skilled person is required to manipulate an ultrasound probe for obtaining appropriate reflections from the aortic bloodflow. This is often difficult, if not impossible, to do through continuous exercise. Usually the patient is required to stop exercising and stop breathing while the probe is manipulated, both of which are considered undesirable for a quality exercise tolerance test. Moreover, the physiques of some patients simply prevent a doppler echocardiogram from being obtained.
Impedance cardiology offers an attractive alternative to doppler echocardiography and MUGA ejection fraction for noninvasively monitoring global ventricular performance. Moreover, because the impedance methodology is electrode-based, it can be combined with electrocardiography for simultaneously obtaining information about both electrical and mechanical activity of the myocardium for assessing cardiac condition and diagnosing coronary artery disease.